Some generous defending gifted the Welsh club three goals in 11 first-half minutes and there was no coming back.
The Millers remain six points above the dropzone and embroiled in a relegation scrap.
Here are five things we learned from the game.
Defending like that could get you relegated
All three of Cardiff's goals could have been avoided, but they came gift-wrapped with a bow for Russell Slade's men. Bruno Ecule Manga was given the freedom of the penalty area to head home a corner - his side's first foray into the box - while although Federico Macheda's second was a quality finish, the hosts squandered a chance to clear it in the build-up. That was also the case with the third as two attempted clearances went awry allowing Conor McAleny to make it three. It was 11 minutes of madness and it lost Rotherham the game.
It was also put a further dent on the goal difference which is plummeting faster than the mercury in the arctic. The Millers have conceded 12 goals in their last four games and scored only three. With defending like that on show the relegation fight is nowhere near won.
Cardiff didn't even have to play well
There's been enough times this season where Rotherham have had to hold their hands up and say they were beaten by a better team on the day. Well that wasn't the case on Tuesday night and that makes it all the more frustrating. Cardiff were no great shakes and were there for the taking, especially in the first 20 minutes when the only work Millers keeper Adam Collin had to do was field some testing Kirk Broadfoot back-passes. But the generosity of the home defence allowed the Bluebirds a three-goal lead and they were not going to give that up.
Even with that advantage, Cardiff did not kick on and look to put the game to bed. Instead they put two banks of four behind the ball and invited Rotherham to try and break them down. There's no point dwelling on ifs and buts, but we will, if the Millers had scored in that opening 20 minutes, we could be sat here discussing a different outcome completely. Frustrating.
Adam Hammill likes a stepover
As the problem position of right midfield continues, Adam Hammill came off the bench at half-time and made an impact, though also reminded everybody that he can perform a stepover or three. But for all his trickery and showponying, Hammill did at least show an intent to run at players and looked dangerous. He was involved in the Rotherham's two best bits of the second half, sending in a curling shot which Simon Moore had to save before whipping in a cross which Danny Ward headed home five minutes later.
If he can curb that desire to keep hold of the ball for too long, by maybe having a word with Ben Pringle who will always get it into the box at the earliest opportunity, then the Millers could have a genuine threat. It won't be on Saturday, though, as he is ineligible.
We're all Norwich fans
The only real positive to take from Tuesday night was that Millwall also lost, meaning it's as you were at the foot of the table but with one less game to go. So the gap remains six points and with the GD also going in Rotherham's favour last night, it effectively is seven points. On the showing against Cardiff, Steve Evans' men will have to rely on Millwall losing their games to keep that cushion.
Over the next three days, Millers fans should all take on Norwich as their second side. The Canaries, who have won six in a row, play Wigan on Wednesday and then Millwall on Saturday. If by the end of the weekend it's eight in a row, the Millers will be stroking their chins, if the Canaries lose both, it's twitchy bum time.
It was not the magical return for Alex Revell (apart from the scoreline)
While we will never hear a bad word said about Alex Revell and the lung-busting effort he put into the Millers cause, it was pleasing to see him have a largely ineffective game on his return to New York. Yes, he worked hard and put himself about but even on an evening when the Rotherham defence were in such a generous mood, their former team-mate hardly got a sniff. He did have a goal disallowed of course and forced Collin into a decent save in the second half, but on the whole Revell made little impact.
He was warmly-received by the Millers fans before the game and shared hugs with his ex-colleagues after the game, but was unlikely to have gone into the manager's office for a glass of wine.
